Irrigation systems utilize a series of pipes or conduits for conveying water from a water source to a series or network of water emitters or sprinklers. Though at one time metallic pipes were used, these suffered from exterior rust and corrosion, as well as interior fouling, and carried a high materials expense. Coatings were developed which promoted the life of metallic pipes, and the use of large diameter pipes helped alleviate the problems with interior fouling. However, cost remained an issue until polymeric piping was developed. Polymer or plastic piping offered additional benefits. For instance, threading and sealing of plastic joints is simpler and more reliable. One drawback to plastic piping has traditionally been its strength, particularly in localized regions containing stress concentrators or non-uniform mechanical properties.
A common irrigation system component manufactured from plastic materials is a pop-up irrigation sprinkler. This type of sprinkler is typically buried in the ground and includes a stationary housing and a riser assembly mounted within the housing which cycles up and down during an irrigation cycle. During irrigation, pressurized water typically causes the riser assembly to elevate through an open upper end of the housing and rise above the ground level to distribute water to surrounding terrain. The pressurized water causes the riser assembly to travel upwards against the bias of a spring to the elevated spraying position to distribute water to surrounding terrain through one or more spray nozzles. When the irrigation cycle is completed, the pressurized water supply is shut off and the riser is spring-retracted back into the stationary housing.
Pop-up irrigation sprinklers generally include either a spray or rotary nozzle on the riser assembly. Spray riser assemblies commonly have one or more spray nozzles that are fixed relative to the riser assembly and distribute water over a set area. By contrast, a rotary riser assembly commonly includes a rotatable nozzle turret mounted at the upper end of the riser assembly. The turret includes one or more spray nozzles and is rotated through an adjustable water distribution pattern. Rotary sprinklers often include a water-driven motor to transfer energy of the incoming water into a source of power to rotate the turret. During normal operation, the turret rotates to distribute water outwardly over surrounding terrain in an arcuate pattern. In either case, the housing configured to receive the riser assembly generally has either a side or lower inlet for connecting to the input supply water.
Plastic sprinkler housings are typically injected molded, but injection molding techniques can lead, in some cases, to non-uniform mechanical properties in the sprinkler housing, such as a weakened point or weakening feature referred to as a knit line. Specifically, thermoplastic polymers flow at a rate dependent on their temperature and the speed of injection. The polymeric materials are typically forced into a mold with a temperature lower than that of the plastic. Therefore, the plastic begins to cool on contact with the mold. This cooling is most pronounced at the leading edge of the injection flow, and the leading edge begins to solidify and develop a surface texture as it flows through the mold. When two leading edges meet, a line referred to as a knit or mold line may be formed. The plastic material localized at the knit line generally does not tend to form as strong a bond because the polymer may not join and flow as well at this interface. Therefore, the knit line may not bond as well and may be a weaker point in the housing. Though the sprinkler housings may be reinforced with fibers, such as fiberglass, these fibers often do not cross the knit line interface, so the knit line may not realize the benefits of the presence of the fibers. Therefore, the knit line may weaken the sprinkler housing.
Sprinkler housings with side inlets also tend to have a reduced life as compared to sprinkler housings with only a bottom inlet. During use, the maximum stress on the sprinkler housing is generally experienced when the water source is turned on and a pressure surge is transmitted to the sprinkler housing. That is, the pressure directed through the sprinkler housing is not free to simply pass through the housing because any standing water in the sprinkler housing must first be pressurized to overcome its static state. This generates stresses throughout the sprinkler housing. It has been found that sprinkler housings with a side inlet tend to underperform housings without a side inlet. More particularly, the side-inlet sprinkler housing typically has stress concentrations proximate the edges forming the side inlet and most often at the knit line about the side inlet. These stress concentrations can result in fatigue and cracks that originate in these high-stress areas. These cracks can grow resulting in either water leaks or fracture causing the sprinkler housing to be inoperable. Even a small leak can reduce sprinkler performance and waste large amounts of water.